Method for making a mould and for producing an elastomeric skin

ABSTRACT

A method for making a mould ( 26 ) for producing an elastomeric skin which shows a texture. The required negative of this texture is provided on the mould surface by cutting the desired texture in the liner structure ( 24 ) used to form the mould surface; and/or by providing a master model ( 14 ) and by making the liner structure as an impression of the master model ( 14 ). The master model itself is provided with the desired texture by cutting this desired texture in the master model ( 14 ) or by using an elastic sheet material ( 12 ) forming the surface of the master model. The desired texture is either cut in the elastic sheet material ( 12 ) or the elastic sheet material ( 12 ) is made as an impression of a starting material ( 1 ) by moulding a flowable impression material having a Brookfield viscosity at application temperature smaller than 100 Pa·s, preferably smaller than 50 Pa·s, or even lower. By the use of such low viscous impression materials, a texture of a better quality can be achieved on the mould surface.

The present invention relates to a method for making a mould forproducing at least an elastomeric skin in particular by a spray, RIM orslush moulding process, which skin shows a front surface having apredetermined texture being an impression of a desired texture, whichmethod comprises the steps of providing a supporting mould structure;and arranging a liner structure on the mould structure to provide amould surface having a size and a shape corresponding to the size andthe shape of the front surface of the elastomeric skin and showing atexture which is a negative of said predetermined texture.

In a method which is used in practice, the liner structure comprises agalvano plate which is made in by electrodepositing nickel in a nickelplating bath or galvano cell onto a bath model. This bath model is madeby a number of successive impressions from a master model which has asurface with a size and a shape corresponding to the size and the shapeof the mould surface, the master model being more particularly anegative of the mould surface or in other words a positive of themoulded skin. The required texture on the master model is provided byapplying an elastic sheet material, showing the required texture, onto amaster model structure, more particularly onto a so-called ureol model.The elastic sheet is a thermoplastic PVC film having the requiredtexture embossed thereon according to a so-called “präging” processwherein the film is embossed under pressure by means of an embossingroll. The embossing roll is heated somewhat so that the film isplastically deformed without melting the PVC material. The thus embossedPVC films show an elongation at break of 200-250% and a tensile strengthof 8-14 MPa, both measured according to ISO 527-3/2/100, and a tearresistance of 30-55 N/mm, measured according to DIN 53515. In practicethe embossed films are produced in large quantities for use inthermo-forming processes, such as deep-drawn processes, to produce forexample the elastomeric skin of trim parts for automobiles such as dashboards, glove compartments, consoles, door panels, etc.

A drawback of the above described technique is that the quality of thetexture provided on the embossing roll is reduced considerably whentransferring the desired texture from the roll to the thermoplasticfilm. This is for example due to the presence of variations intemperature on the surface of the roll, variations in pressure betweenthe roll and the film, differences in slip between the roll and the filmin the axial direction of the roll and possible damages to the roll. Inpractice, in one particular example of a PVC film embossed with aleather structure, variations in the maximum peak to valley heightsR_(z) (measured as an average in accordance with ISO 4287-1997 and ISO11 562 over a sampling length % c determined in accordance with ISO4288-1996 measured in the same location in different areas with arepeating pattern) from 90 to 140 μm (and usually even greater) andvariations of the gloss of the film from 1.6 to 2.4 have been observed.Such variations result in a lower quality of the moulds and of the partsproduced therein, which consequently have not only worse visual but alsoworse touch characteristics. A further drawback of the existingtechnique is that the installation for embossing a thermoplastic film bymeans of an embossing roll is too expensive for making at the most a fewsquare meters of elastic sheet material required to produce a mastermodel. Moreover, in order to bring the installation to the requiredprocess conditions, some hundreds square meters of film have to beproduced, most of which is waste material.

EP-A-0 342 473 discloses another method for making a mould for producingan elastomeric skin by a spray process. In this known method the linerstructure for the mould is formed by a spray process onto a mastermodel. This European patent application however does not disclose thepresence of a texture onto the master model. Moreover, for applying theliner structure onto the mould structure, the liner structure has to bedeformed considerably so that, in case a texture would be provided onthe master model, this texture would be deformed considerably.

An object of the present invention is therefore to provide a new methodfor producing a mould which enables to transfer the desired texture witha higher accuracy to the mould surface so that parts with a higherquality, such as better visual and touch characteristics, can beproduced in the mould.

To achieve this object the method according to the invention ischaracterised by the features defined in claim 1.

In this method the desired texture can first of all be cut directly inthe liner structure so that no quality is lost at all by any impressionswhich have to be made. Secondly, use can be made of a master modelwherein the desired texture is directly cut. The existing mould makingprocesses enable in this case to make the mould surface with a minimumloss of quality. The existing method for making the mould liner startingfrom a master model uses indeed a gradual deposition process (nickelplating process) and moulding processes wherein silicone mouldings(impressions) are made in a closed mould so that high qualityimpressions can be made. Instead of cutting the desired texture in themaster model it can be cut in an elastic sheet used to form the surfaceof the master mould. When starting from a starting material which showsthe desired texture or wherein the desired texture is cut to produce theelastic sheet material the loss of quality of the texture is reduced inthe method according to the invention compared to the above describedexisting technique due to the fact that use is made of an Impressionmaterial for moulding the elastic sheet which is flowable and which hasmore particularly a Brookfield viscosity of 100 Pa·s or less. The PVCfilm used in the existing technique becomes on the contrary not flowablebut is only heated so that it can be plastically deformed.

The present invention also relates to a method for producing at least anelastomeric skin showing a front surface having a predetermined texture,which method comprises the steps of providing a mould having a mouldsurface having a size and a shape corresponding to the size and theshape of the front surface of the elastomeric skin and showing a texturewhich is a negative of said predetermined texture, and producing theelastomeric skin in particular by a spray, RIM or slush moulding processagainst said mould surface. This method is characterised in that use ismade of a mould made in accordance with the above described mould makingmethod according to the invention.

Due to the use of such moulds, elastomeric skins showing a more uniformtexture or a texture of a better quality, i.e. a texture whichapproaches more the desired texture from which the mould making processstarts, can be achieved.

The invention moreover relates to a mould for producing at least anelastomeric skin showing a front surface having a predetermined texture,which mould comprises a mould structure and a liner structure arrangedin the mould structure providing a mould surface having a size and ashape corresponding to the size and the shape of the front surface ofthe elastomeric skin and showing a texture which is a negative of saidpredetermined texture. The mould according to the invention ischaracterised in that it is made in accordance with a method accordingto the mould making method of the present invention. As explainedalready hereabove, such moulds enable to produce elastomeric skinsshowing a more uniform texture or a texture of a better quality.

Other particularities and advantages of the invention will becomeapparent from the following description of some particular embodimentsof the method and the mould according to the present invention. Thereference numerals used in this description relate to the annexeddrawings which illustrate the different steps of the mould makingprocess according to a particular embodiment of the invention andwherein:

FIG. 1 shows a schematic sectional view on a roll provided with a sleeveof a starting material wherein the desired texture is being cut by meansof a laser;

FIG. 2 shows a schematic sectional view on a vacuum table wherein thestarting material is positioned and wherein an impression material ispoured to produce a first impression of the starting material;

FIG. 3 is a view similar to FIG. 2 wherein the impression material isset to form an elastic sheet material as first impression;

FIG. 4 shows a sectional view of the elastic sheet material;

FIG. 5 illustrates on a larger scale the quality of the texture on thefirst impression, i.e. onto the elastic sheet material, when using theprior art “präging” process and the moulding process according to thepresent invention, the figure illustrating both the contour of thetexture onto the starting material and the contour of the texture on theelastic sheet material, the texture on the “praged” prior art PVC filmbeing illustrated in dashed lines;

FIG. 6 shows a sectional view of a master model structure, moreparticularly of a so-called ureol model;

FIG. 7 shows the ureol model illustrated in FIG. 6 having appliedthereon the textured elastic sheet material to form the master model;

FIG. 8 illustrates the master model of FIG. 7 arranged together with asupport shell in a mould to produce an intermediate silicone impression;

FIG. 9 illustrates the silicone impression of FIG. 8 which is arranged,together with the support shell, in a reversed position in a furthermould wherein a gel coat and a glass fibre-epoxy layer are moulded ontop of the silicone impression to produce a so-called bath model;

FIG. 10 illustrates the bath model immersed in a nickel plating bathwherein a nickel shell is deposited by electric current onto the bathmodel, and

FIG. 11 illustrates the nickel shell arranged on a supporting mouldstructure to produce the mould.

In the method according to the invention illustrated in the figures adesired texture is first of all cut in a starting material 1. Thestarting material 1 is for example a silicone rubber sleeve or a“Novotek” sleeve which is applied onto a roll 2. By means of amechanical milling device, but preferably by means of a laser 3 withlaser beam 4, the desired texture is cut in the starting material 1. Thelaser cutting technique is already known per se and disclosed forexample in U.S. Pat. No. 5,759,473 so that it will be described in greatdetail. The roll 2 is for example rotated while the laser beam 4 isguided over the peripheral surface parallel to the axis of rotation insuch a manner that essentially the entire peripheral surface is treated.During this movement of the laser beam 4 relative to the surface of theroll 2, the intensity of the laser beam 4 is controlled as a function ofa control signal which may be obtained by scanning the surface of apatterned original or an artificially designed texture can be developed.In case of an existing texture, for example a leather, wood, stone orany other texture, the surface may be scanned optically or mechanically,and an electric signal is generated in accordance with the scanningresults and may be used directly to control the position and intensityof the laser. A preferred alternative consists however in temporarilystoring the electric signal obtained by scanning the patterned original.In this way the scanned texture can be processed to modify it.

In the prior art method, the laser cut roll is used to emboss athermoplastic PVC film at a relatively high output by a continuouscalendering process.

In the method according to the present invention, at the most a fewimpressions have to be made of the starting material 1, but theseimpressions have to be of a better quality. As illustrated in FIG. 2,the sleeve of starting material 1 can be removed from the roll 2 andpositioned in a flat state on a vacuum table 5. This vacuum table 5 hasa flat bottom 6 provided with holes 7 ending in a chamber 8 wherein avacuum is created by means of a vacuum pump 9 so that the sheet ofstarting material is kept in a flat position. Referring to FIG. 3, aflowable impression material 10 is poured in a next step onto thestarting material 1 and is scraped off by means of a scraper 11. Afterhaving allowed the impression material 10 to set, the sheet material 12produced thereby is demoulded. This sheet material has for example anaverage thickness (calculated by dividing its volume by its surfacearea) of between 20 μm and 5 mm, and preferably between 0.5 mm and 2 mm,and is illustrated in FIG. 4.

As impression material for producing the sheet material 12, use is madeof a material which produces an elastic sheet material, showing inparticular an elongation at break, measured according to ISO527-3/2/100, of at least 20%, preferably of at least 50% and morepreferably of at least 100%. Such an impression material may be athermoplastic material such as PVC, TPO or TPU, a flowable siliconerubber or a reactive mixture such as a polyurethane reaction mixturecomposed of an isocyanate and a compound, in particular a polyalcohol ora polyamine, comprising groups which are reactive to the isocyanatecompound. An essential feature of the impression material is that, atthe temperature at which it is applied onto the starting material, itshould have a Brookfield viscosity smaller than 100 Pa·s, in particularsmaller than 50 Pa·s, so that it can copy the texture of the startingmaterial sufficiently accurately. The thermoplastic materials shouldtherefore be heated to a sufficiently high temperature and/or contain asufficient amount of plasticizers in order to be suited for use asimpression material. It should be noted that materials with the highestviscosities are in the form of a paste which requires some smearingaction and/or some pressure in order to fill the surface relief of thestarting material sufficiently completely. The term “flowable material”is therefore to be understood in the present specification as embracingnot only materials which can be poured simply by gravity out of acontainer but also materials to which some pressure has to be applied inorder to flow. In the method according to the invention, preference isgiven to materials which have even a lower viscosity at applicationtemperature, more particularly a Brookfield viscosity smaller than 30Pa·s, preferably smaller than 20 Pa·s and more preferably smaller than10 Pa·s. Silicone rubbers, having a viscosity of for example 20 to 25Pa·s, but especially polyurethane reaction mixtures, having an initialviscosity which is smaller than 10 Pa·s, or even smaller than 5 Pa·s,are preferred.

Examples of suitable polyurethane reaction mixtures can be found inEP-B-0 379 246, especially in Examples 1 to 18 thereof, which areincluded herein by way of reference. For the present invention, somemodifications can be applied to these examples. In view of the fact thatfor the present invention the curing time is of minor importance, thecatalysts could for example be omitted or reduced and less reactivecompounds could be used, for example a longer polyol and/or a polyolhaving a lower functionality, for example a minimum functionality oftwo, so as to achieve an even more elastic polyurethane. Instead of analiphatic isocyanate, it is on the other hand possible to use a morereactive aromatic isocyanate.

The advantage of the use of a low viscous impression material, inparticular a low viscous reaction mixture, is illustrated in FIG. 4.This Figure illustrates the profile of the desired texture cut in thestarting material 1 and the profile of the resulting texture on theelastic sheet material moulded on top of the starting material. With alow viscous material less than 5% more particularly only 1 to 3% of thequality, especially of the maximum peak to valley heights R_(z), will belost by making the impression due to the fact that the small recesses ofthe fine texture will usually not be filled completely. With theexisting mechanical embossing process on the contrary, even the largerrecesses of the coarse texture will not be completely filled resultingin a considerable loss of quality. In the example illustrated in FIG. 5,the fine texture superimposed onto the basic coarser texture is forexample completely lost.

In the method according to the invention, a more uniform elastic sheetmaterial can be made on the basis of the laser cut starting materialthan in the existing calendering process. In case a repeating pattern isprovided on the elastic sheet material 12, the maximum peak to valleyheights R_(z) (measured as an average in accordance with ISO 4287-1997and ISO 11 562 over a sampling length λ_(c) determined in accordancewith ISO 4288-1996) measured in the same location (where the elasticsheet material shows the same surface profile) in the different areaswith repeating pattern vary for example within a range of less than 15%,in particular even less than 10%, above and below the average of themeasured peak to valley heights in the different areas. When using suchhigh quality textured sheet materials for making mould for adjoiningtrim parts, for example for a dash board and for a glove compartmentcover or for an air bag, there will be no noticeable contrast betweenthe texture qualities of these parts.

For making the elastic sheet material 12 the flowable impressionmaterial 10 can be poured onto the starting material 1 as describedhereabove, at least when the impression material shows a sufficientlylow viscosity. Instead of pouring the impression material 10, it couldalso be sprayed, for example by applying the technique disclosed for apolyurethane reaction mixture in EP-B-0 303 305. The impression materialcould even be sprayed directly onto the roll 2, i.e. without removingthe starting material therefrom, so that no vacuum table would berequired. When the impression material is more viscous, the texturedsurface can still be copied sufficiently accurately by smearing thematerial onto this textured surface. Moreover, also for less viscousmaterials, it is possible to exert a pressure onto the impressionmaterial so that it penetrates even better in the smallest recesses ofthe textured surface, for example by injecting the flowable materialunder pressure in a closed mould cavity. Of course, the applied pressureshould not be too large to avoid deformations of the textured surface.In practice, a RIM (reaction injection moulding) process should forexample be suitable, in particular a polyurethane RIM process.

In the method illustrated in the figures, the elastic sheet material 12is made as a direct impression of the starting material 1. In a variantembodiment, one or more intermediate impressions could be made by meansof impression materials showing the same properties as describedhereabove so that the quality of the texture is very little affected bymaking the different intermediate impressions. Instead of using astarting material 1 wherein the desired texture is cut, it is alsopossible to start from an existing, in particular natural texturedmaterial, such a for example natural leather, a wooden panel, a piece ofstone, a solidified sand surface, a metal piece, etc. This texturedmaterial can be applied onto the vacuum table or onto another mouldingdevice so that the elastic sheet material 12 can be moulded directlyonto this textured material.

In another preferred embodiment, the texture on the elastic sheetmaterial 12 is not provided as an imprint of a starting material buttthe desired texture is directly cut in the sheet material 12, preferablyby means of a laser 3. It is clear that in this way no loss of qualitycan occur. Cutting the desired texture in the elastic sheet material canbe done in the flat or curved state of the sheet material 12 or, in thesame way as described for the starting material 1, by making a sleeve ofthe sheet material 12 and applying it around a roll 2.

In the method illustrated in the figures, the elastic sheet material 12is used to cover or wrap at least portions of a master model structure13 in order to achieve a master model 14. If necessary two or moreelastic sheets 12 can be made by successive mouldings starting from asame starting material. The master model 14 shows a surface having asize and a shape corresponding to the size and the shape of the surfaceof the mould to be produced. By a corresponding size and shape is meantin the present specification a size and a shape which are substantiallyidentical to the size and the shape of the mould surface or a size and ashape which are substantially identical to the size and the shape of anegative impression of the mould surface. Consequently, when making thefurther impressions to achieve the mould surface, no deformations willoccur.

When applying the elastic sheet material 12 onto the master modelstructure 14, it has to be stretched to conform to the shape of thisstructure, which is possible due to the elastic nature of the elasticsheet material 12.

Instead of making the master model 14 starting from a master modelstructure 13 and a textured elastic sheet material 12 or pieces thereof,it is also possible to cut the desired texture directly into the mastermodel 14, preferably by means of a laser. In this way no loss of qualitycan occur in any previous moulding steps. However, the movements whichhave to be performed by the laser and/or by the master model will bemore complex due to the three-dimensional shape of the master model.

In a next step, illustrated in FIG. 8, an intermediate impression ismade, more particularly a silicon model 15. The master model 14 is placein this step in a mould 16 and a support shell 17 is positioned on topof the master model 14 so that a mould cavity is formed. This mould isfilled with a silicone composition to produce the silicon model 15. Asillustrated in FIG. 9, this silicon model 15 is used to produce, as afurther intermediate impression, a bath model 18. For making this bathmodel 18, the silicon model 15 is positioned together with the supportshell 17 in a further mould 19 and a gel coat layer 20 and a glass fibrereinforced epoxy layer 21 are moulded on top of the silicon model 15.

As illustrated in FIG. 10, the thus produced bath model 18 issubsequently immersed in an electrolytic bath 22, more particularly in anickel plating bath or galvanic cell. This bath 22 is filled with anelectrolyte solution and two anodes 23 are arranged in the bath. Byapplying a voltage between the bath model 18 and the anodes 23 anelectric current flows in the bath and a metal layer 24, in particular anickel shell, is gradually deposited onto the bath model 18. This nickelshell 24, showing an imprint of the original desired texture, forms aliner structure which is arranged in a last step onto a supporting mouldstructure 25 to produce a mould 26, the mould surface of which is formedby the liner structure 24. Instead of using a nickel plating bath, it isalso possible to deposit the metal layer onto the bath model by means ofa metal vapour deposition process, more particularly by a nickel vapourdeposition process. Since such vapour deposition processes are known perse, they will not be described in further detail in the presentspecification.

In the above described process, the different impressions can be madestarting from the master model 14 with such an accuracy that when themaster model is provided with a texture showing a repeating pattern, themaximum peak to valley heights R_(z) (measured as an average inaccordance with ISO 4287-1997 and ISO 11 562 over a sampling lengthλ_(c) determined in accordance with ISO 4288-1996) measured in the samelocation in the different areas with repeating pattern vary within arange of less than 15%, preferably less than 10%, above and below theaverage of the measured peak to valley heights in the different areas.When the master model is made starting from an elastic sheet which isstretched in certain areas, the maximum peak to valley heights R_(z)have of course to be determined in areas of the mould surface where theelastic sheet was not, or substantially not stretched, since thestretching of the elastic sheet will have an effect on this value. Thematerials used for making the different intermediate impressions willnormally have a viscosity in the same range as the viscosities describedhereabove for the impression materials for making the elastic sheetmaterial 12, unless a higher pressure can be used to mould theseintermediate impressions. Also the electrolytic or vapour depositionprocesses produce only a negligible loss of texture quality.

Instead of providing the texture on the master model 14 either byapplying a textured elastic sheet material 12 or by cutting the desiredtexture in the master model 14, the desired texture could also be cut inany of the intermediate impressions, in particular in the silicon model15 or in the bath model 18, which has then to be considered as themaster model in accordance with the definitions given in the annexedclaims. On the other hand, further intermediate impressions can be madein order to produce the liner structure 24.

Instead of starting from a textured master model, it is further alsopossible to cut the desired texture directly into the liner structure24, and this preferably also be means of a laser. Due to the threedimensional shape of the liner structure 24, this requires more complexmovements of the laser and/or of the liner structure. An importantadvantage is however that no quality loss is produced by making anyimpressions and that undesired deformations by stretching of an elasticsheet material can be avoided.

Instead of a metal liner structure, it is also possible to make theliner structure from an elastic material. Such an elastic liner ispreferably preshaped so that it has a shape and a size correspondingsubstantially to the shape and the size of the mould surface of themould obtained when arranging the liner structure onto the mouldstructure. The liner structure may consist in particular of a rubbermembrane, more particularly of a silicone rubber membrane, moulded inaccordance with the shape of the mould surface. In such an elasticliner, the desired texture can be cut by means of a laser requiringagain more complex movements of liner and/or laser to cut the texture.The use of an elastic liner structure enables however to turn the linerstructure, or certain areas thereof, inside out so that concave portionsof the liner structure can be reached better by the laser beam. If theelastic liner is elastic enough, it could further be stretched, or atleast certain portions thereof to bring the liner into a more regularflat or curved state so that the texture can be cut more easily, inparticular by means of the laser.

The moulds made in accordance with the methods described hereabove areused to produce an elastomeric skin by a method wherein a flowable skinmaterial is applied onto the mould surface so that a predeterminedtexture is formed on the front side of the skin. Examples of suchmethods are spray processes, RIM processes and powder or liquid slushmoulding processes. In the method according to the invention preferenceis given to a spray or a RIM process wherein a liquid polyurethanereaction mixture is applied onto the mould surface. Compared to slushmoulding processes wherein the thermoplastic moulding material has to beheated to a temperature which is generally higher than 200° C. andwherein the molten material has subsequently to be cooled down ratherquickly, a spray or RIM mould is usually heated to a much lowertemperature and does not need to be cooled down so that it is lesssubjected to thermal stresses. In practice, the mould surface of suchmoulds can therefore be made of materials which can be given a moreaccurate or a finer texture.

1. A method for making a mould for producing at least an elastomericskin in particular by a spray, RIM or slush moulding process, which skinshows a front surface having a predetermined texture being an impressionof a desired texture, which method comprises the steps of: providing amould structure; and arranging a liner structure on the mould structureto provide a mould surface having a size and a shape corresponding tothe size and the shape of the front surface of the elastomeric skin andshowing a texture which is a negative of said predetermined texture,characterised in that the liner structure is provided with said negativetexture: by cutting the desired texture in the liner structure; and/orby providing a master model showing a surface having a size and a shapecorresponding to the size and the shape of said mould surface and bymaking the liner structure as a direct impression of the master model oras an impression of one or more intermediate impressions of the mastermodel, the direct and any intermediate impressions being made bygradually depositing impression material on the master model and/or onany intermediate impression and/or by a moulding process wherein aflowable impression material is applied onto the master model and/oronto an any intermediate impression and wherein the flowable impressionmaterial is allowed to set to produce the impression, the master modelbeing provided with the desired texture by cutting this desired texturein the master model or an elastic sheet material is applied onto amaster model structure to form the surface of the master model, thedesired texture being cut in the elastic sheet material before applyingit onto the master model structure or the elastic sheet material is madeas a direct impression of a starting material or as an impression of oneor more intermediate impressions of a starting material, which startingmaterial shows the desired texture or the desired texture being cut inthe starting material, the direct and any intermediate impressions beingmade by a moulding process wherein a flowable impression material havinga Brookfield viscosity at application temperature smaller than 100 Pa·s,preferably smaller than 50 Pa·s, is applied onto the starting materialand/or on any intermediate impression and wherein the impressionmaterial is allowed to set to produce the impression.
 2. A methodaccording to claim 1, characterised in that the desired texture is cutby means of a laser beam.
 3. A method according to claim 1,characterised in that the liner structure is made of an elasticmaterial, in particular of a rubber, according to a shape and a sizecorresponding substantially to the shape and the size of the mouldsurface and the desired texture is cut in this liner structure.
 4. Amethod according to claim 1, characterised in that the liner structureis provided with said negative texture by making it as a direct orindirect impression of said master model.
 5. A method according to claim4, characterised in that the liner structure is made of a metal or metalalloy which is gradually deposited on the master model or on said or oneof said intermediate impressions, in particular by metal plating in anelectrolytic cell or by a metal vapour deposition process, the metalbeing preferably nickel.
 6. A method according to claim 4, characterisedin that said master model is made by applying said elastic sheet ontothe master model structure, the elastic sheet showing an elongation atbreak, measured according to ISO 527-3/2/100, of at least 20%,preferably at least 50% and more preferably at least 100%.
 7. A methodaccording to claim 4, characterised in that said master model is made byapplying said elastic sheet onto the master model structure, the elasticsheet being made by moulding said impression material by spraying,pouring, injecting or smearing it in a layer onto the starting material,the impression material being preferably flowable molten PVC, flowablesilicone rubber or a liquid polyurethane reaction mixture, preferencebeing more particularly given to the use of a liquid polyurethanereaction mixture.
 8. A method according to claim 4 characterised in thatsaid master model is made by applying said elastic sheet onto the mastermodel structure, the elastic sheet being made as a direct or indirectimpression of said starting material, the impression material ormaterials used to make said direct and any intermediate impressionshaving a Brookfield viscosity at application temperature smaller than 30Pa·s, preferably smaller than 20 Pa·s and more preferably smaller than10 Pa·s.
 9. A method according to claim 4 characterised in that saidmaster model is made by applying said elastic sheet onto the mastermodel structure, the elastic sheet being provided with the desiredtexture by cutting this texture in the elastic sheet or in said startingmaterial, the desired texture being preferably cut in the elastic sheet.10. A method for producing at least an elastomeric skin showing a frontsurface having a predetermined texture, which method comprises the stepsof: providing a mould having a mould surface having a size and a shapecorresponding to the size and the shape of the front surface of theelastomeric skin and showing a texture which is a negative of saidpredetermined texture; and producing the elastomeric skin in particularby a spray, RIM or slush moulding process against said mould surface,characterised in that use is made of a mould made in accordance with amethod according to any one of the previous claims.
 11. A methodaccording to claim 10, characterised in that the elastomeric skin isproduced by applying a polyurethane reaction mixture either by a sprayor a RIM process against the mould surface and by allowing the reactionmixture to cure, an in-mould coating, in particular an in-mould paintbeing optionally first applied against the mould surface.
 12. A mouldfor producing at least an elastomeric skin showing a front surfacehaving a predetermined texture, which mould comprises a mould structureand a liner structure arranged on the mould structure providing a mouldsurface having a size and a shape corresponding to the size and theshape of the front surface of the elastomeric skin and showing a texturewhich is a negative of said predetermined texture, characterised in thatthe mould is made in accordance with a method according to claim
 1. 13.A mould according to claim 12, characterised in that the liner structurehas a texture showing a repeating pattern, the maximum peak to valleyheights Rz (measured as an average in accordance with ISO 4287-1997 andISO 11 562 over a sampling length λc determined in accordance with ISO4288-1996) measured in the same location in the different areas withrepeating pattern varying within a range of less than 15%, preferablyless than 10%, above and below the average of the measured peak tovalley heights in the different areas.